Charles L. Braun

Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production

The lowest charge‐transfer excited state (CT1) of electron donor–acceptor crystals or polymers is demonstrated to be a plausible precursor of free charge carriers when such materials are photoexcited. Rate constants for the dissociation of charge–transfer states are formulated for two approximate descriptions of CT1: classical ion pair and Wannier exciton. The electric field dependence of the dissociation rate constant is postulated to be given by Onsager’s 1934 theory (O‐34) of ion pair dissociation. This formulation of CT1 dissociation obviates the need to invoke electron–hole ‘‘thermalization’’ lengths of 2 to 3 nm in order to explain free charge carrier formation in donor–acceptor materials.

Temperature dependence of intrinsic carrier generation in anthracene single crystals

The pulse technique is used to measure free carrier yields in anthracene single crystals as a function of temperature, photon energy, and applied field. Field dependence plots, measured over the temperature range 200–300 K, exhibit slope‐to‐intercept ratios described by S/I=3.2×10−5 (298/T)2 cm/V. This result is in quantitative agreement with that predicted by Onsager’s theory of geminate recombination. Activation energy (Ea) results are reported over a wide range of photon energies. We find that Ea is essentially constant (Ea=0.090 eV) over the photon energy range 4.4–5.2 eV and again constant (Ea=0.068 eV) over the range 5.4–6.2 eV. This observation is consistent with autoionization from two spectroscopically observed states: system II (1B2u) in the lower energy range, and system III (1B1u) in the higher energy range. The average thermalization lengths that would be associated with these Ea values are in the range 10–70 A depending on the form chosen for the distribution of charge pair configurations.

Intrinsic photoconduction in anthracene single crystals: Electric field dependence of hole and electron quantum yields

For electric fields up to 2 × 104 V/cm applied normal to the a b plane of virgin anthracene crystals, both the hole and electron photocarrier quantum yields increase linearly with field. Field‐dependence plots exhibit a slope to intercept ratio of 3.1 × 10−5 cm/V, a value which is in quantitative agreement with that predicted by Onsagers theory of geminate recombination. In nonvirgin crystals, sharply reduced photocarrier quantum yields at low fields are found to result from recombination of free carriers with oppositely charged trapped carriers left behind in the excitation region from previous experiments. The free carrier‐trapped charge recombination rates are found to be diffusion controlled.

Geminate charge recombination in the photoionization of N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) in various solvents

The photoionization of N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) dissolved in n‐hexane (HEX), 2,2,4‐trimethylpentane (TMP), and tetramethylsilane (TMS) is studied using measurements of single‐photon photoconductivity. Quantum yields for free carrier production—extrapolated to zero applied field—are reported for the energy range 4.4–7.5 eV. The applied electric field dependence of the quantum yields is measured up to fields approaching 2×107 V/m and interpreted via Onsager’s theory of geminate recombination. The low‐field, slope/intercept predictions of the theory are obeyed quantitatively for HEX and TMP as solvent but fail by 25% for TMS. Various distribution functions for the separations of geminate pairs are used to generate predicted field dependence curves, which are compared with the high‐field data. A broad distribution of thermalization lengths is definitely required to fit the TMP and TMS data, but the TMS result is suspect because of the observed failure of Onsager theory in that solvent. ...

The photoconduction threshold in anthracene single crystals

We report the excitation spectrum for intrinsic photoconductivity measured with unpolarized light incident on the 001 face of an anthracene single crystal. Over the photon energy range 3.75 to 4.4 eV, the room temperature (near 295 K) quantum yield of free carriers rises by a factor of 103. The activation energy for free carrier production rises steeply between 3.87 and 4.06 eV and then levels off. These results are compared with recent work by Lyons and Milne and some discrepancies are noted.

Rotation and vibration spectra for the H2O dimer: Theory and comparison with experimental data

The ground state binding energy (BE), rotational and vibrational energy levels, and line strengths for radiative transitions between some of these energy levels are calculated for the [H2O]2 molecule. These quantities are computed for three intermolecular potentials published for the [H2O]2 molecule, two of which were calculated by MO SCF techniques and one which was derived from an empirical point charge model for the water molecule. The value of BE calculated for two of the potentials is approximately 6 kcal/mole, and for the third is approximately 3 kcal/mole. The total concentration of [H2O]2 in equilibrium with H2O vapor is calculated for the sets of energy levels determined for these potentials. Results are compared with available experimental data. Using calculated values of line strengths and experimental data for the integrated absorptions for two rotational transitions, an independent value is deduced for the equilibrium concentration of [H2O]2 in fair agreement with values calculated from two o...

Picosecond geminate charge recombination: the dependence of reaction kinetics on initial pair separation

Abstract The recombination kinetics of geminate electron-cation pairs in liquid hexane depends on the photoionization wavelength used for charge pair creation. Pairs generated by two-photon ionization of anthracene with 355 nm light exhibit a first half-life for recombination of 108 ps at 191 K; 266 nm light, 930 ps. Longer recombination times are attributed to a greater electron thermalization length in the liquid. The initial pair is studied both by the temperature dependence of the escape probability and by kinetic calculations based on the Smoluchowski diffusion equation.

Intrinsic Photoconductivity in Naphthalene Single Crystals

For naphthalene single crystals, we observe charge carrier generation which exhibits quadratic intensity dependence for photon energies from 3.8 to 5 eV. Our experiments appear to rule out involvement of triplet excitons, and we attribute ionization to the mutual annihilation of two singlet excitons. The room temperature rate constant for singlet–singlet annihilation is calculated to be γ ≤ 1.7 × 10−9cm3sec−1 and that for carrier generation is β = 2.3 × 10−14cm3sec−1. These values are compared with those for anthracene where the latter values are revised based on a recent estimate of the intrinsic anthracene singlet exciton lifetime. For photon energies above 5 eV, we observe increased carrier yield and decreased intensity dependence exponent and tentatively ascribe these results to onset of single‐photon ionization at about 5 eV.

Advances in the transient dc photocurrent technique for excited state dipole moment measurements

Recent advances in the transient dc photocurrent technique for measuring excited state dipole moments, developed in our group, are discussed. A variety of approaches with detailed analyses of their advantages and disadvantages including cell design, circuit construction tricks, the data acquisition procedure, calibration, and the theoretical treatment of different conditions, are presented. Sensitivity, time resolution limitations, and newly developed features, such as the signal’s dependence on light polarization as well as charge separation at interfaces are outlined. Dipole moments of a few molecules (diphenylcyclopropenone, bianthryl, dimethylaminonitrostilbene, Coumarin 153, and fluoroprobe) suitable for calibration purpose are reported—some of them for the first time.

Field and Temperature Dependent Recombination in Anthracene

The quantum efficiency of photoconductivity in anthracene depends on electric field and temperature. Analysis of these results strongly suggests that the generation of free holes and electrons can be described by a model in which, following the initial ionization, most electron-hole pairs are thermalized while still bound by their mutual Coulomb field. The electron and hole may then either recombine or separate as free carriers as they diffuse subject to their mutual Coulomb field and the applied field. Interpreted in the light of this model, the dependence of free carrier quantum yield on temperature and photon energy indicates that the primary ionization process involves energy relaxation and autoionization rather than direct ionization.